"50%" will also work in place of "center" for x and y axis. "0" for "left" on the x-axis and for "top" on the y-axis. And "100%" for "right" on the x-axis and for "bottom" on the y-axis. ... so `background: #f00 url(image.jpg) repeat-y 50% 0;` is the same as above but slightly less bytes.
Also the CSS spec lists the shorthand properties in this order: ([background-color] [background-image] [background-repeat] [background-attachment] [background-position]) which is why I tend to put the 'repeat' value before the 'position' values.
Check out the spec here: http://www.w3.org/TR/CSS2/colors.html#background — Andy Ford about 8 years ago